Subsea raw water injection facility

ABSTRACT

A subsea raw water injector including a pump which is connected to an injection string and a filter connected to an inlet of the pump to remove particulates from the surrounding sea water. The filter is in the form of an inclined tube settler disposed such that particulates separated from sea water flowing through it are discharged to the seabed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a subsea raw water injection facilityfor injecting sea water into an oil bearing formation.

There is a demand for new techniques to reduce the cost of explorationand production activity in off-shore oil fields such as the North Sea.As oil is discovered in ever deeper water, the costs of and risksassociated with fixed platforms supporting production equipmentincrease. Thus anything which can be done to reduce the amount ofequipment that has to be supported above the surface of the sea isuseful.

2. Discussion of the Related Art

It is conventional practice to inject sea water into subsea oil bearingformations to assist with the process of sweeping oil from the formationand maintaining the pressure of the formation. Generally an oilproduction platform is provided with a processing plant to which seawater gathered from close to the sea surface is delivered. Theprocessing plant typically comprises in series a coarse filter in theform of a screen, a fine filter, typically a body of sand, achlorination unit, an oxygen scavenging unit and a de-oxygenation unit.The water is chlorinated to avoid biological activity and de-aerated toprevent rapid corrosion of pipes used to deliver processed water to aninjection string leading to the subsea formation. The various sea waterprocessing units are heavy and bulky and therefore supporting them abovethe surface of the sea is a significant problem, particularly in deepwater.

International Patent Specification No. WO 94/29222 describes a sea waterinjection system in which the relevant processing units are mounted onthe seabed. In the described system, a pump connected to an injectionstring is located within an enclosure the walls of which are porous. Thepump is energized to draw sea water through the porous wall andappropriate arrangements are made to chlorinate the water as it passesthrough the wall. Particulates separated from the sea water passingthrough the wall will build up on the outside of that wall but it isbelieved that turbulence within the sea water will be sufficient toprevent the filter defined by the wall from being blinded.

The arrangement described in the published patent specification has itsattractions in terms of simplicity but the concentration of particulatesin sea water adjacent the seabed will be sufficiently high duringperiods of turbulence to make it difficult to predict the performance ofthe filter. Given the cost implications of an installed system failingthere is a preference for using tried and test filtration systems whichhave a positive mechanism for discharging particulates separated fromthe pumped sea water.

A filter system is available which efficiently separates both mineralparticulates such as sand and organic particulates which have neutralbuoyancy. The known system incorporates an ejection mechanism which isperiodically actuated so as to discharge separated particulates from thesystem. The use of such a system in the seabed environment is consideredto be acceptable in terms of performance, but there is great concernabout the long term viability of the particulate ejection system giventhat it is expected that large quantities of particulates will beseparated and therefore the ejection system will have to be operated atregular intervals, for example many times each day.

SUMMARY OF THE PREFERRED EMBODIMENTS

It is an object of the present invention to provide a subsea injectorsystem incorporating a filter which obviates or mitigates the problemsoutlined above.

According to the present invention, there is provided a subsea raw waterinjection facility comprising a pump which in use is positioned on theseabed and connected to an injector string of an oil bearing formation,and a filter connected to an inlet of the pump, the filter being open tothe sea and being arranged to remove particulates from sea water drawnthrough it by the pump, wherein the filter comprises an inclined tubesettler disposed such that particulates separated from sea water flowingthrough it are discharged to the seabed.

A large array of individual settling tubes can be supported on anassembly also incorporating the pump and ancillary filtration equipmentdesigned to removed particulates of neutral buoyancy. The flow ratethrough each tube can be sufficiently low to ensure a very highpercentage of mineral particulates are discharged from the tube andtherefore do not contribute to the load on the ancilliary filtrationequipment which is primarily provided to remove neutral buoyancyparticulates.

Inclined tube settlers are well known, having been first introduced inthe 1940s. They are used in for example water treatment plants toseparate coagulated/flocculated material from processed water. In theknown tube settlers, separated particulates are delivered to a dischargemechanism. In the subsea environment, such a discharge mechanism wouldcarry with it the potential problems referred to above with regard tothe available filtration and discharge mechanisms. The present inventionis based firstly on the realization that tube settlers are well suitedto separating out the highly variable concentrations of mineralparticulates which occur in sea water adjacent the seabed, and secondlyon the realization that, providing the tubes are appropriatelypositioned, separated particulates can be allowed to accumulate beneaththe tubes until turbulent conditions arise which result in the dispersalof those particulates. Simply by ensuring that the seawaterinlet/particulate discharge ends of the tubes are at a sufficient heightabove the seabed to prevent those ends becoming buried in accumulatingsand ensures reliable continuous operation.

Preferably the tubes of the tube settler are formed from an array ofsuperimposed corrugated sheets with the corrugations of adjacent sheetsoffset and secured together.

The injection facility may comprise a frame which supports the pump anddefines a cover to protect the facility against dropped objects, thetube settler being in the form of arrays of tubes distributed beneaththe edges of the cover.

The downstream ends of the tubes may be connected to a manifold which isconnected to the pump inlet, means being provided to deter marine lifefrom entering the manifold. Marine life may be deterred by chlorinatingwater within the manifold. The slow current flow through each tubeavoids the irreversible ingestion of marine animals capable of swimmingagainst that current and out of the facility through the seawaterinlet/particulate discharge end of the tube. A simple device such as anet may be disposed across the upstream ends of the tubes to prevent theentry of all but small marine animals. In addition or as an alternativethe tubes may be vibrated, for example by coupling the tubes to avibrating component such as the pump. Such vibration assists in theself-cleaning of particulates from the tubes.

Preferably an ancillary filter is provided between the pump and the tubesettler to remove neutral buoyancy particulates, the ancillary filterbeing provided with means for periodically discharging accumulations ofparticulates to the surrounding sea water.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is schematic perspective view of a subsea injection facility inaccordance with the invention;

FIG. 2 is a schematic representation of part of one of the tube settlersincorporated in the embodiment of FIG. 1;

FIGS. 3 and 4 illustrate alternative arrangements for interconnectingcomponents of the settler of FIG. 2;

FIG. 5 is a cross-section of a tube used to test the efficiency of asettler having a shape as shown in FIG. 2;

FIG. 6 is a graph comparing the performance of the tube of FIG. 5 with atube of circular cross-section; and

FIG. 7 is a graph illustrating the performance of the different lengthsof the tube of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the illustrated structure comprises a tubular framedefining outwardly splayed legs 1, a rectangular base 2 and verticallegs 3. The legs are connected to the underside of a cover 4 whichserves to protect the assembly from dropped objects.

The frame supports a pump 5 an inlet which is connected to an inletstrainer 6 which in turn has an inlet 7 coupled to a manifold (notshown) beneath the cover 4. The strainer 6 is adapted to remove neutralbuoyancy particulates from sea water flowing through it, theparticulates being periodically removed by a discharge device 8 throughan cutlet nozzle 9. The manifold beneath the cover 4 is connected to thedownstream ends of a tube settler defined by six arrays of tubes 10disposed beneath the edges of the cover 4.

The tube settler removes a large proportion of the mineral particulateswhich may be in the sea water within which the injection facility isimmersed. Neutral buoyancy particles will of course pass unimpededthrough the tube settler but these will be removed by the strainer 6 andperiodically discharged through nozzle 9. As a result the lifeexpectancy of the strainer and the associated ejection equipment will begreatly enhanced as compared with a system in which no tube settler wasprovided.

The arrays of tubes may be manufactured in any appropriate manner. Testshave shown however that a particularly efficient performance can beobtained using a settler fabricated as illustrated in FIG. 2. As shownin FIG. 2 the corrugated sheets 11, 12, 13 and 14 are stacked one fromthe other with the corrugations offset and welded together, one of thewelds being located for example at the point indicated by numeral 15.The corrugations may be of any suitable geometric shape, for examplecorresponding to a sine wave, the objective being to maximize the widthof each tube whilst minimizing the height and maximizing the number oftubes packed within a unit volume and yet retaining an acceptable flowcross-section. The assembly of tube arrays from superimposed corrugatedsheets results in a very robust and yet easy to manufacture structure.

As shown in FIG. 3, the interconnection between adjacent corrugationscan be effected by welding a projection 16 formed on one corrugatedsheet within a groove formed in the adjacent corrugated sheet. As analternative to the arrangement of FIG. 3, the projection 16 on one sheetcould be received in a groove defined between two spaced apartprojections 17 on the adjacent sheet as shown in FIG. 4.

FIG. 5 shows a cross-section of a tube formed from interconnecting twostrips of material cut from a sheet of corrugated plastics, each striphaving a width corresponding to one “waveleugth” of a generallysine-wave shape. Each corrugation has a peak to peak amplitude of 1.8 cmand a wavelength of 7.5 cm such that the width of the tube is 7.5 cm andits depth is 3.6 cm. With this configuration a relatively restrictedrapidly tapering portion is formed at each side of the tube and thiscould conceivably cause a problem if particles were to build up in thisarea, but this problem could be overcome for example by adopting theconfiguration of FIG. 4. All the tests referred to below were howeverconducted on the basis of a structure as illustrated in FIG. 5.

Tests were conducted with tubes having cross-sections as shown in FIG. 5and lengths of 1.0 m and 1.8 m. The tubes were inclined at 45° to thehorizontal and were operated at various flow rates. The tubes weretested to determine their performance in separating out silica sand andAccrington Blue particles with a size range of from 63 to 90 and from 90to 106×10⁻⁶ meters respectively. Water was circulated through the tubes,the water having a solids concentration of 5.0 grams per liter and beingvigorously agitated.

Tests were also conducted with tubes of circular cross-section having aninternal diameter of 4.3 cm. These were used to provide a basis forcomparing the performance of conventional tubular settlers with thoseincorporating tubes as shown in FIG. 5.

FIG. 6 plots removal efficiency against flow rate for the circular tube(lower curve) and the tube of FIG. 5 (upper curve), both tubes being 1 mlong. These results show that removal efficiency falls significantly atflow rates above 0.25 m³ per hour and falls much more rapidly at flowrates of between 0.25 and 0.3 m³ per hour in the case of the tubularcross-section.

It is believed that the superior performance of the tube according toFIG. 5 is due to the relatively larger ratio of wetted perimeter to thecross-section and settling area provided by the cross-section of FIG. 5as compared with a circular cross-section. In particular, the tube asshown in FIG. 5 has a settling depth of only 3.6 cm whereas that of thecircular section tube of equivalent cross-sectional area is 4.3 cm. Ashorter settling depth reduces the settling time required and henceincreases the tendency of particles which have slow settling velocitiesto settle

FIG. 7 graphically represents the relative performance of two tubes bothhaving the cross-section of FIG. 5 but one having a length of 1.0 m(lower curve) and the other having a length of 1.8 m (upper curve).These results clearly suggest that a longer tube significantly improvesremoval efficiency. It will be noted that for a tube having thecross-section of FIG. 5 and a length of 1.8 m, if the tube is operatedat a flow rate of at most 0.40 m³ per hour the removal efficiency ismore than 90%.

Longer tubes not only provide a larger and longer settling area, theyalso overcome the problem associated with the alteration of particletrajectories due to increased flow rates. This was readily apparent byobservation of the turbidity of water within the tube which was 1.8 mlong, as the water appeared relatively turbid at a distance of 1 m fromthe tube inlet but was fairly clear approaching the tube outlet.

On the basis of the above results, a 200 m³ per hour seabed filtrationfacility having a minimum removal efficiency of 90% would require 500tubes having a cross-section as shown in FIG. 5 and a length of 1.8 m. Aremoval efficiency of 99% could be achieved with 1000 tubes. It may beadvisable to provide a baffle or screening arrangement adjacent the tubeinlet to equalize flows resulting from turbulence in the sea.

What is claimed is:
 1. A subsea raw water injection facility comprisinga pump positioned on a seabed and connected to an injector string of anoil bearing formation, and a filter positioned on the seabed andconnected to an inlet of the pump, the filter being open to the sea andbeing arranged to remove particulates from sea water drawn through thefilter by the pump, wherein the filter comprises an inclined tubesettler disposed such that particulates separated from sea water drawnthrough the tube settler are discharged to the seabed.
 2. An injectionfacility according to claim 1, wherein the tube settler comprises tubesformed from an array of superimposed corrugated sheets, wherein thecorrugations of adjacent sheets are offset and connected together.
 3. Aninjection facility according to claim 2, wherein downstream ends of thetubes are connected to a manifold connected to the inlet of the pump,and further comprising means to deter marine life from entering themanifold.
 4. An injection facility according to claim 3, wherein themeans to deter marine life from entering the manifold comprises means tochlorinate water within the manifold.
 5. An injection facility accordingto claim 2, further comprising a net for obstruction access by marinelife to upstream ends of the tubes.
 6. An injection facility accordingto claim 1 or 2, wherein the pump and tube settler are supported on aframe including a cover to protect the facility against dropped objects,the tube settler comprising arrays of tubes distributed beneath edges ofthe cover.
 7. An injection facility according to claim 1 or 2, furthercomprising means for vibrating the tube settler.
 8. An injectionfacility according to claim 1 or 2, further comprising an auxiliaryfilter connected between the pump inlet and tube settler, the auxiliaryfilter being arranged to remove neutral buoyancy particulates andincluding means for periodically discharging accumulations ofparticulates.
 9. An injection facility according to claim 1, furthercomprising a net for obstructing access by marine life to upstream endsof the tube settler.
 10. An injection facility according to claim 1,wherein downstream ends of the tube settler are connected to a manifoldconnected to the inlet of the pump, and further comprising means todeter marine life from entering the manifold.
 11. An injection facilityaccording to claim 10, wherein the means to deter marine life fromentering the manifold comprises means to chlorinate water within themanifold.